The invention relates to a data storage medium having an information carrier wound onto a winding core in a spiral manner for optically readable information units.
DE 298 16 802 describes a data storage medium having an information carrier, wound in a plurality of plies onto a winding core in a spiral manner, for optically readable information units. The information carrier may comprise a polymer film, with an adhesion layer being located between each pair of adjacent plies. Information can be written to this data storage medium by locally heating the polymer film by means of a write beam of a data drive, as a result of which the refractive index and thus the reflecting power (reflectivity) change locally at the interface of the polymer film. This may be detected by means of a read beam in the data drive. By focusing the write beam or read beam, information may be specifically written to or read from a preselected ply of the information carrier. The winding core may be optically transparent and may have a recess in its central area that serves to accommodate the read/write device of a data drive. The read/write device is moved relative to the data storage medium, while the data storage medium is stationary, so that the data storage medium need not be balanced to take account of a rapid rotational motion.
In the previously known data storage medium, it proves to be disadvantageous that the inner end of the information carrier, when it is wound onto the cylindrical winding core, forms a type of projection, to which the following plies of the information carrier have to adapt, so that the result is deviations from an ideal spiral form. In particular, the radial spacing of the inner plies of the information carrier from the center of the winding core changes rather abruptly in the area of the projection. When data are being read out or written in, this can lead to difficulties in tracking the focus of the read beam or the write beam.
It is an object of the invention to provide a data storage medium having an information carrier wound onto a winding core in a spiral manner for optically readable information units from which the information can be read out without problems.
This object is achieved by a data storage medium having the features of claim 1. Advantageous refinements of the invention emerge from the dependent claims.
The data storage medium according to the invention has an information carrier wound onto a winding core in a spiral manner for optically readable information units. The winding core is configured in a spiral manner on its outer contour and has a step, whose height is matched to the thickness of the information carrier, the inner end of the information carrier resting on the winding core at the step or in the area of the step.
The fact that the outer contour of the winding core is already configured in a spiral manner, and the inner end of the information carrier rests on the winding core at the step or at a relatively short distance therefrom, the information carrier, at the start of its second winding, encounters a largely smooth surface, so that the radial spacing of the information carrier from the center of the winding core increases gradually over the entire length of the information carrier (in such a way that it largely corresponds to an ideal spiral shape) and not abruptly. The invention therefore has a particularly beneficial effect on data storage media in which the information carrier is wound in a plurality of plies, and in this case the inner plies in particular profit. If the read beam orxe2x80x94if the data storage medium can also be writtenxe2x80x94the write beam scans the information carrier, it is accordingly not necessary to displace the focus suddenly at specific locations, for which reason higher reading speeds andxe2x80x94if appropriatexe2x80x94writing speeds can be achieved than in the case of the previously known data storage medium.
In a preferred embodiment of the invention, the spirally configured outer contour with the step is formed integrally on the winding core.
In an alternative configuration, the winding core has a component similar to a cylinder and a separate component, which at least partly surrounds the component similar to a cylinder and on which the spirally configured outer contour with the step is formed. In this case, the separate component can be flexible, at least during the production of the winding core, can have a basic shape similar to a wedge, the length of the wedge being less than or equal to the periphery of the component similar to a cylinder, and the height of the wedge corresponding to the height of the step, and can be placed onto the component similar to a cylinder. With the aid of the separate component, therefore, the spiral-like outer contour of the winding core, deviating from the cylindrical shape, is formed. The length of the wedge can be less than the periphery of the component similar to a cylinder; however, the result is then greater deviations from the ideal spiral shape. Preferably, the separate component and the component similar to a cylinder are adhesively bonded to each other during the production of the winding core.
In a preferred embodiment of the invention, the winding core is optically transparent and has a recess in its central area. In this case, it is possible to arrange in the recess in the central area of the winding core a read device and, optionally, a write device of a drive tuned to the data storage medium and to move it relative to the data storage medium, while the data storage medium is stationary, for the purpose of reading and/or writing information. A stationary data storage medium has the advantage that it does not need to be balanced to permit high rotational speeds, which has a beneficial effect on the production costs.
The information carrier preferably comprises a polymer film whose refractive index can be changed locally by heating. Material considered for the polymer film comprises, for example, polymethylmethacrylate (PMMA) or biaxially oriented polypropylene (BOPP). If polypropylene, following extrusion to the film, is pretensioned in two planes, a high inherent energy is stored in the material. In the case of local heating, by means of a write beam, for example, there is a severe change in the material by reformation, and this is so even when a relatively small amount of energy is deposited per unit area. In this way it is possible, for example, to achieve a change in the refractive index of approximately 0.2 over an area for one stored information unit with a diameter of approximately 1 xcexcm.
The polymer film may be assigned an absorber which is set up at least partly to absorb a write beam and to emit the generated heat at least partly, locally, to the polymer film. The absorber comprises, for example, dye molecules which are present in the polymer film or in an adhesion layer adjacent to the polymer film, and permits local heating of the polymer film, sufficient to change the refractive index, for a relatively low write beam intensity.
As already mentioned, the advantages of the invention come to bear in particular if the information carrier is wound onto the winding core in a plurality of plies. The polymer film is therefore preferably wound up in a plurality of polymer film plies, through which information can be written into a preselected polymer film ply or can be read from a preselected polymer film ply. There may be an adhesion layer provided in each case between adjacent polymer film plies in order to fix the polymer film plies to one another. For example, 10 to 30 polymer film plies may be wound atop one another, or else a greater or lesser number. At a polymer film thickness of between 10 xcexcm and 100 xcexcm, preferably below 50 xcexcm or around 35 xcexcm, the information on different polymer film plies can be separated from one another with good resolution by means, for example, of read/write devices which are known from DVD technology. An adhesion layer may, for example, have a thickness in the range between 1 xcexcm and 40 xcexcm, preferably below 25 xcexcm or around 2 xcexcm. A suitable adhesion agent comprises, for example, an acrylate adhesive which is free from air bubbles and which is crosslinked, for example, chemically or by irradiation with UV or electron beams. If an adhesion layer is arranged between adjacent polymer film plies, the height of the step on the winding core should correspond approximately to the thickness of the polymer film plus the thickness of the adhesion layer.
Preferably, the refractive index of the adhesion layer differs only slightly from the refractive index of the polymer film, in order to minimize disruptive reflections of the read beam or of the write beam at a boundary between a polymer film ply and an adjacent adhesion layer. It is particularly advantageous if the difference in the refractive indices is less than 0.005. Any difference in the refractive indices may, however, be utilized for the purpose of formatting the data storage medium.